The existence of triplet code was simply an assumption till 1961, when Nirenberg and Mathaei proved its existence by experiments. They were able to synthesise artificial, mRNA which contained only one nitrogenous base, ie, uracil. This synthetic poly U sequence was placed in a cell free system containing protein synthesising enzymes (extracted from bacterium E. coli) and 20 amino acids togeth;- with necessary ATP. During the process, a sma_ polypeptide molecule was produce which was formed by the linking of phenylalanine. This suggested that UUU is the code for phenyl alanine. Nirenberg got Nobel Prize for ha contributions.